The present invention relates to a diaphragm carburetor for an internal combustion engine, especially for a two-stroke engine having in a carburetor housing an intake channel for guiding combustion air into the internal combustion engine. The intake channel has a venturi section and upstream of the venturi section a throttle valve. A fuel-filled control chamber is provided which is connectable by an inlet valve operated by a control diaphragm to the fuel supply channel. A first fuel channel extends from the control chamber to a main outlet valve in the venturi section.
In hand-guided working tools such as motor chainsaws, cutters, trimmers etc., it is common to employ two-stroke engines as a drive means. In order to provide the two-stroke engine with the required fuel/air mixture, diaphragm carburetors are known which comprise in a carburetor housing an intake channel which supplies the combustion air into the internal combustion engine. The intake channel has a venturi section. Upstream of the venturi section a throttle valve is arranged with which the flow cross-section for the intake air can be adjusted as a function of the throttle actuation. The throttle valve cooperates with a full load stop which is unchangeable. Because of manufacturing and assembly tolerances, the flow cross-section for the combustion or intake air is not exactly identical; usually, considerable differences are observed, especially because of deviations of the throttle valve angle in the open position in the range of .+-.3.degree..
Furthermore, such a diaphragm carburetor has a fuel-filled control chamber which can be connected by a fuel inlet valve controlled by the control diaphragm to the fuel supply channel. A first fuel channel extends from the control chamber to a main outlet valve that is arranged within the venturi section of the intake channel. A second fuel channel extends from the control chamber to an idle valve which is arranged in the area of the throttle valve of the intake channel. Within the passage way to the main outlet valve as well as in the passage way to the idle valve, control valves in the form of adjusting screws having ends in the form of a valve cone are provided so that the fuel ratio for the respective load condition, i.e., idle condition or full load condition, can be adjusted to the desired .lambda. value. However, with this construction even minimal changes or manufacturing tolerances cause considerable fuel/air ratio changes which remain within the limits set by respective regulations, but the corresponding specifications can be reached only with very high manufacturing and adjusting expenditures. Due to the minimal flow cross-section of these screw valves, multiple disturbing influences are present, for example, due to temperature-depending viscosity changes of the fuel and the soiling of the valve so that a substantial change of the fuel/air mixture composition will result which affects the operational behavior of the combustion engine.
It is therefore an object of the present invention to provide a diaphragm carburetor for an internal combustion engine of the aforementioned kind in which in a simple manner with utmost precision the mixture composition of fuel and air can be adjusted so that a reduced band width of the .lambda. range for full load is achieved.